Aromatic nitrocompounds are compounds where one or more nitro-groups are bound to a carbon atom in an aromatic ring. Typical aromatic nitrocompounds are e.g. nitrobenzene, nitrotoluene, nitroxylene and dinitrotoluene. Aromatic nitrocompounds can be hydrogenated to correspnding aromatic amines.
It is well known that many of the relatively high boiling aromatic amines can be prepared by liquid-phase hydrogenation of the corresponding aromatic nitrocompound in the presence of a catalyst, either with or without the use of a solvent.sup.1. Early catalytic hydrogenations were performed in stirred batch reactors. In this known method the aromatic nitrocompound is first dissolved into a suitable solvent and pumped into a batch reactor containing the catalyst. The reactor which is equipped with a stirrer is pressurized by hydrogen gas. After the reaction is completed, the product can be separated from the solvent.
More recently batch processes have been replaced by continuous processes. A continuous liquid-phase process is illustrated by the process for diaminotoluene.sup.2,3. According to the process, dinitrotoluene (DNT) is catalytically hydrogenated at 150-200 bar and about 100.degree. C. A solution, which contains about 25 wt-% dinitrotoluene dissolved in methanol, hydrogen and Raney-nickel, is pumped through a series of reactors. Reactors are equipped with internal circulation to make the reaction more complete. After the reaction is completed, the pressure is reduced and the excess hydrogen removed in a gas-liquid separator and recycled to the beginning of the process. Catalyst is then removed and recycled, after which the solvent is distilled and recycled. The water is removed in the dehydration column. The product purity is more than 99-% pure diaminotoluene.
Low molecular weight alcohols, particularly methanol or ethanol, are the most commonly used hydrogenation solvents for hydrogenation of aromatic nitrocompounds. Other solvents, like acetic acid, ammonia, benzene, glycerol ethylene glycol, hydrochloric acid, sulfuric acid or water can be used.
It is well known that the solubility of hydrogen into liquid-like solvents is extremely small. Due to the this low hydrogen solubility and slow transfer to the catalyst surface where the reaction product is formed, the reaction in liquid-phase is relatively slow, and continuous reactors could not always be used effectively. Several mechanical solutions have been introduced. One solution is to use a spray column where the solvent, nitroaromatic compound and catalyst are sprayed with the hydrogen gas.
It is known that aromatic nitrocompounds can be hydrogenated in the vapor phase. In this method a vaporized nitroaromatic compound and hydrogen gas flow through the catalyst bed forming the product. The reaction is usually very fast and high conversions are obtained with a single pass. This method is continuous and in principle simple. A method for producing aniline from nitrobenzene in a trickle-bed reactor is described in the literature.sup.4. Nitrobenzene is first vaporized and mixed with a 200% excess of hydrogen gas. The hot gaseous mixture flows upward into the reduction chamber containing the copper-silica catalyst. The reaction is very fast at 270.degree. C. and 2-3 bar. After leaving the reactor, the reaction mixture containing aniline, hydrogen and water is cooled. Excess hydrogen is first removed after which water and aniline are separated. Finally aniline is purified to a 99% product.
The catalytic hydrogenation of aromatic nitrocompounds to aromatic amines can be done in the vapor phase, provided that he boiling point of the compound is low enough and the starting material and the product are thermally stabile. These limitations mean that only relatively simple aliphatic and aromatic nitrocompounds, such as nitrobenzene and nitroxylene, can be hydrogenated in the vapor phase.